1 Answer
1

Not being able to ban a sender does not mean that you can not delete the message.

So, proceed like this:

Ban in batches, for instance 5 by 5, and ignore the batches that fail

When you only have a few left, ban one by one. Now you only have the ones whose ban really fails

Select them all and press "Delete". These users will not be banned but at least they will not prevent you from banning the others in the future. Also, even if these few users are not banned, they are not allowed either so if they post again they will have to go through moderation, hopefully with a successful ban this time.